The invention relates to a gas passage with a selectively acting penetration surface. Moreover, the present invention relates to a process for manufacturing a selectively acting gas passage. Gas passages of this kind are employed in measurement or analytical instruments, for example. It is aimed at admitting lighter gases with a high preference and heavier gases with a lower preference into a measuring instrument, for example.
As disclosed in DE-A-43 26 265, for example the entire contents of which are incorporated by reference, a leak detector is known, the test gas detector of which is equipped with a selective admission system. Helium is used as the test gas. The selective admission comprises a penetration surface formed by a separating wall or a diaphragm, in which the permeability for helium is by many decades higher than the permeability for other gases. For example, quartz, quartz glass (SiO.sub.2), Pyrex glass or also polymer diaphragms made of FEP, for example, offer these properties.
Generally, measuring and analytical instruments should be of high sensitivity and offer a short response time. In a test gas detector according to incorporated DE-A-43 26 265 it is therefore a design target to let as much helium as possible penetrate the penetration surface in a time which is as short as conceivable. This is only possible when on the one hand the penetration surface is as large as possible and when on the other hand it is as thin as possible. This is opposed by the fact that generally the penetration surface will be exposed to a differential pressure. The internal volume of the measuring instrument is evacuated whereas atmospheric pressure prevails outside. Because of the load imposed by the differential pressure there are limitations as to the size and thickness of the penetration surface.